Germany's coalition deal must win support of SPD party members or risk falling apart.
After agreeing a deal on Wednesday the decision now goes to the party's - over 400, 000 - members who will vote in a ballot on whether they back the union.
The SPD announced on Wednesday that it will go into government with Angela Merkel's CDU and its Bavarian Sister Party the CSU.
The SPD came under pressure to form a government with Merkel after a four month period of political instability. But many younger members are unhappy with party's soon to be ex-leader Martin Schulz's decision to join forces. They feel instead a time in opposition could revive the party who have been polling at an historic low.
The SPD's youth wing Jusos - an abbreviation for Jungsozialisten, or young socialists- is the youth wing of the left-leaning SPD, Germany’s oldest and second-most-powerful political party. Its 28 -year-old leader Kevin Kühnert has been leading the grassroots rebellion of the left.
'A lot of the SPD members, especially the younger ones that I know feel like they're being betrayed... yes the SPD will get another four years in government but it may actually not be a majority party in another 8-12 years’ time, or even at the next election' said Christoph Nguyen, political science researcher at Berlin's Free University.
Many SPD members are still bruised from the last time the party joined forces with Angela Merkel, which resulted in their worst election result since the 1930s.
In recent weeks thousands joined the grassroots initiative, NoGroKo, opposing the coalition. Some left-leaning political analysts are comparing the initiative to the Momentum movement in the UK.
Kevin Kühnert, the leader of the youth wing of the Social Democrats said the result of the party vote could go either way: "This is not easy to tell, I think the [outcome of] the ballot is completely open. We as Jusos will throw ourselves into the debate during the next three weeks, make our viewpoint clear, the party leadership will do so as well and we will hopefully experience good, fruitful discussions, and in the end, 450,000 members have their say."
Germany has already broken its post second world war record for time elapsed between the September election and signing in a new government, which will not happen for several weeks.